The subject invention relates to an apparatus and method for printing images. More particularly, it relates to an apparatus and method for printing images such as postal indicia printed by postage meters to evidences that appropriate postage has been paid on a mail piece. The recorded amount is decremented until either sufficient funds do not remain to pay additional postage or the meter is reset to record payment of an additional amount to a postage service.
As evidence that postage has been paid, i.e. that the prepaid amount stored in the meter has been properly decremented, a postage meter will print a postal indicia on a mail piece. A typical indicia is shown in FIG. 1. Indicia 10 includes fixed elements such as city name 92, state 94, meter identification 18, an arbitrary and complex elements 20, and variable elements such as date 22 and, of course, postage amount 24 represented by the indicia. Typically, indicia 10 have been printed by complex mechanical rotary or flat bed printing elements which include a fixed printing element for printing fixed information and adjustable elements for printing adjustable information. It is apparent that postage indicia 10 has an intrinsic value equal to amount 24 in that it is equivalent to a stamp having that value. For this reason city and state identifications 92 and 94 meter identification 18 and date 22 are provided to facilitate detection of fraudulent use of meters and arbitrary and complex design 20 is included to make counterfeiting of indicia more difficult.
Such meters have been highly successful and are presently used to account for the expenditure of billions of dollars of postage in the United States alone. However, as postage rates have increased so has the value of postal indicia and this, together with the need of postal services for increased revenue, has created a demand for a postage meter which is more secure against fraud and counterfeiting. To achieve this it has been proposed that the amount of variable information in the indicia be increased, particularly that the indicia include encrypted information which would vary from indicia to indicia so that counterfeit indicia could not be produced without knowledge of the encryption method and key used.
Additionally it is very advantageous to provide users of postage meters with the ability to print ads or slogans along with the postal indicia, and it would be highly desirable to enable these users to easily vary such ads or slogans.
For these and other reasons it has long been recognized that it would be highly desirable to provide a postage meter which used digital, or matrix, printing techniques so that the desired indicia including any ads or encrypted information, could be printed under direct control of a postage meter CPU.
One particularly desirable form of digital printing which has been proposed for use in a postage meter is ink jet printing, and particularly piezoelectric ink jet printing. One such postage meter is described in commonly assigned, co-pending U.S. patent application Ser. No. 554,179, filed: Nov. 6, 1995, for: MAIL HANDLING APPARATUS AND PROCESS FOR PRINTING AN INDICIA COLUMN - BY - COLUMN IN REAL TIME, by: Arsenault et al. (E -394) In this meter a printhead having a number of ink jets is oriented transversely to the printing direction and is moved over the mail piece to print the indicia. Because the printhead has a density of 80 jets per inch while a vertical resolution of 240 dots per inch is desired for the indicia the ink jet described in the above referenced application prints the indicia in 3 interleaved passes. (Other approaches to increased the vertical resolution of ink jet printing include angling the head from the vertical and the use of staggered multiple heads. However these approaches obviously increase both the cost and complexity of the printing mechanism.)
While the postage meter described in the above referenced application does successfully overcome many of the problems of prior art postage meters, some problems remain.
Since the printhead is moved over the mail piece in 3 separate passes clearly the mail piece must be stopped during printing while it would be desirable for the mail piece to move continuously through the meter. Also, even if multiple or angled printheads were used to provide adequate vertical resolution problems remain combining adequate resolution in the printing, or horizontal, direction together with a high through put rate.
Also, since the product of the horizontal resolution and the through put speed, (i.e. the relative speed of the mail piece with respect to the printhead) is equal to the printhead frequency (i.e. the frequency with which a jet can emit discrete ink droplets). Since there is an inherit maximum printhead frequency for each printhead improvements in the horizontal resolution can only be obtained by trading off throughput.
Another problem with ink jet printing apparatus in a postage meter is that the movement of paper mail pieces through a meter generates substantial amounts of paper dust which is believed to have a very adverse effect on ink jet printheads.
Thus it is object of the subject invention to provide an improved apparatus and method for printing images such as postal indicia.